Permeation for a single-well two-barrier model. Steady-state ion flux and occupancy of the binding site were computed for the case of two permeant ions present at physiological concentrations on either side of the gating pore (in mM [K]_e = 3, [K]_i = 103, [Na]_e = 112, [Na]_i = 7). The “open-channel” current–voltage relation that is in absence of gating is compared in A for the assumption of independence (Scheme 1, dashed line) or competition by Na⁺ and K⁺ for the single binding site (Scheme 3, solid line). When voltage-dependent gated accessibility of the pore is included (B), the current–voltage relation has strong inward rectification and the distinction between Schemes 1 and 3 is no longer discernable.